In a screw compressor and such that compress a gas by making a pair of male and female rotors to be engaged with each other, a typical cause of performance degradation is an internal leakage. The internal leakage is a phenomenon in which a compressed gas flows backwards from a compression chamber formed between the male and female rotors to a compression chamber of lower pressure. Since suction of the gas is inhibited by the internal leakage and power loss occurs by recompression of a leakage gas, performance of the screw compressor and such is degraded. In the screw compressor and such, a series of continuous contact points are formed between the male and female rotors. The series of continuous contact points is called a meshing seal line. The meshing seal line has a function of sealing the compressed gas. A length of the meshing seal line is preferred to be short from a view point of reducing the internal leakage of the gas. Trials for suppressing the gas leakage from the meshing seal line are made by shortening the length of the meshing seal line formed between the male and female rotors as much as possible as a measure against the internal leakage.
As a second problem, there is a problem of a “blowhole”. In a screw rotor in which a female rotor has an addendum outside of a pitch circle and a male rotor has a dedendum inside a pitch circle, a blowhole is formed. The blowhole is formed among male and female rotors and a cusp line where bores formed in a casing are crossed. Through the blowhole, a gas leakage occurs. Formation of the blowhole is described with reference to FIG. 8. FIG. 8 is a view illustrating a cross-section perpendicular to an axis. In a casing bore b formed inside a casing 100 of the screw compressor, a male rotor 102 and a female rotor 104 are provided. The male rotor 102 and the female rotor 104 rotate in directions of arrows about a center-of-rotation OM and about a center-of-rotation OF, respectively. In the figure, PM represents a pitch circle of the male rotor 102, and PF represents a pitch circle of the female rotor 104.
An internal wall of the casing bore b also has a gas sealing function of the compression chamber by being in contact with the male and female rotors. Hereinafter, a line of intersection between the internal wall of the casing bore b and the cross-section perpendicular to the axis is called a tip seal line c. A meshing seal line s formed between the male rotor 102 and the female rotor 104, and the tip seal line c formed at a rotor outer peripheral part are not connected and are discontinuous. The discontinuous part is called a blowhole, and is literally an open ceiling section. The blowholes are formed at two positions that are a suction side blowhole B1 and a compression side blowhole B2. The suction side blowhole B1 is formed between an upper cusp point Pk1 and the meshing seal line s. The compression side blowhole B2 is formed between a blowhole side closest point Ps of the meshing seal line s and a lower cusp point Pk2.
It is the compression side blowhole B2 which causes a problem from a viewpoint of performance of the screw compressor.
FIG. 9 illustrates a shape of the compression side blowhole B2. The compression side B2 is formed among the lower cusp line k2, a male rotor side blowhole contour R1 formed by a line of intersection between a tooth face of the male rotor 102 and a blowhole plane including the blowhole side closest point Ps and the lower cusp line k2, and a female rotor side blowhole contour R2 formed by a line of intersection between a tooth face of the female rotor 104 and the blowhole plane. Normally, an area of a cut end cut by the blowhole plane is a representative area of the compression side blowhole B2. This is applied also to the present description. FIG. 9 is a drawing viewed from a direction indicated by an arrow A, and is a drawing in which the blowhole plane is projected onto a plane including a y-axis in FIG. 8. FIG. 10 is a view illustrating a shape of the meshing seal line s viewed in the direction indicated by the arrow A. In the figure, ΔLs represents part of the meshing seal line s corresponding to one tooth.
The applicant previously proposed a configuration of a screw rotor capable of reducing an area of the blowhole (Patent Document 1). The invention is to make a cross-section shape perpendicular to the axis from an addendum top center of the female rotor to a pitch circle on a side advancing against a rotation direction to be formed of three or more arcs, which reduces the blowhole area.